1. Field of the Invention
The present invention relates to a method of and to a device for monitoring the wear of at least one sliding electrical contact of at least one electrically conducting brush, sliding over at least one electrically conducting track which can move with respect to the brush, or vice versa, in a relative rotational or linear motion.
The invention also relates to the use of the method of monitoring the wear of a sliding electrical contact to the state-dependent and/or predictive maintenance of a device which includes at least one such contact between at least two electrically conducting elements, one being able to move with respect to the other.
2. Description of the Prior Art
It is known that many electrical devices and machines generally include several sliding electrical contacts, of the general type mentioned hereinabove, each consisting of the interaction of a brush, of the carbon type or of the type having a wiper wire or wires, which is fixed to a brush-holder support, with a track which is fixed to a track-carrying support and has an electrically conducting coating, for example made of gold or silver, over which the brush slides at a defined speed and in a defined direction of relative movement of the brush over the track because of the fact that the two supports are such that one can move with respect to the other.
Thus, in particular, there are many rotating electrical machines, such as electric motors, alternators, generators, rotating convertors and rotating commutators, in which the sliding electrical contacts are rotary contacts and the electrically conducting tracks are made in the form of rings, the number of rings and the number of brushes being chosen depending on the electrical power or on the number of control or measurement signals to be transferred between the two, brush-holder and ring-holder, supports, one of which, called the rotor, is mounted so as to rotate, generally by means of rolling bearings, over the other, called the stator.
It is known that with regard to the two electrically conducting elements, namely a brush and a ring, one being able to move relative to the other, which together form a sliding electrical contact, it does not matter whether the brush is on the rotor and the ring is on the stator, or the other way round.
Electrical devices and machines also exist which are equipped with sliding electrical contacts for which the brushes and the tracks are in relative linear translational motion with translational speeds, excursions and frequencies, In particular in linear commutators, which may be constant or may vary depending on the applications of such linear commutators.
Likewise, in rotating electrical machines, in particular rotary commutators, the speed and/or direction of rotation may be constant or may vary.
It is also known that the electrical devices and machines having sliding contacts, such as those mentioned hereinabove, equip many stationary or movable installations, motors, landborne, seaborne or airborne vehicles, and in particular aeroplanes and helicopters, on which these devices and machines are currently subjected to preventive or programmed maintenance which, for safety reasons, is particularly rigorous, restricting and expensive.
By way of example, current helicopters equipped with electrical systems for the de-icing or anti-icing, using Joule-effect heating, of the blades of their rotors are equipped with rotating electrical commutators both on the main rotor and on the tail rotor. The main function of such a rotating commutator is to send one or more electrical signals between the on-board electrical mains, on a fixed part, namely the structure of the helicopter, via the stator of the commutator, to a rotating part, namely the blades, via the rotor of the commutator. These electrical signals comprise power signals, for the electrical supply of the heating system resistance elements of the de-icing or anti-icing systems, and may comprise signals for controlling actuators, regulators and other electrical components mounted on the blades.
An additional function of such a rotating commutator may be to send one or more electrical measurement signals between the rotating part, namely the blades, via the rotor of the commutator to the fixed part, namely the structure of the helicopter, via the stator of this commutator.
Because of the friction between the fixed and moving conducting elements forming each sliding contact, these commutators suffer wear and are consequently subjected to periodic maintenance checks and actions, the periods of which vary depending in particular on the current output and on the linear or tangential speed of the linear or rotary contact and on the technology employed to produce the conducting coating of the tracks or rings and to provide the interaction between the latter and the brushes, so as to allow the current to flow in an optimum manner.
Typically, programmed maintenance of such rotating commutators may comprise a check, using a maintenance unit which includes the necessary measurement means, of the electrical continuity of the sliding contacts after a prescribed number of hours in service, for example every 1600 hours, in the case of the commutator of the main or tail rotor, in addition to removing any dust from the rings and brushes of the rotating commutator in the case of the tail rotor (because its speed of rotation is greater than that of the main rotor) after another prescribed number of hours in service has elapsed, for example every 400 hours in operation.
This results in high running and maintenance costs, because the helicopters are grounded for periodic inspections of the commutators, because of instances of unjustified removal of the commutators in order to check them, after a report on their good working order, which burdens the management of the stocks, and because of the considerable amount of equipment and human resources necessary, which result in a high number of maintenance hours for each flying hour of the helicopter.